(1) Field of the Invention
The present invention relates to a semiconductor device using a Group III-V nitride compound semiconductor and a method for fabricating the same.
(2) Description of Related Art
Compounds made of Group III elements, boron (B), aluminum (Al), gallium (Ga) and indium (In), and a Group V element, nitride (N) and represented by the general formula, BwAlxGayInzN (wherein w+x+y+z=1 and 0≦w, x, y, z≦1) are Group III-V nitride compound semiconductors, i.e., the so-called Group III-V nitride semiconductors. Group III-V nitride semiconductors have advantages such as wide band gaps and associated high breakdown voltages, high electron saturation velocities and high mobilities, and also high electron densities in heterojunctions. In view of the above, Group III-V nitride semiconductors are being researched and developed for the purpose of applying them to short-wavelength light-emitting elements, high-power and high-frequency elements, high-frequency and low-noise amplifying elements, or the like.
In order to enhance characteristics of semiconductor elements made of such Group III-V nitride semiconductors, it is necessary to maximally reduce the contact resistance of each semiconductor element. When current is carried by electrons, an ohmic electrode need be formed from outside in the region where electrons are conducted (hereinafter, referred to as an “electron channel”). In this case, it is especially significant to reduce the contact resistance of the ohmic contact.
A known method for forming an ohmic electrode (see, for example, Japanese Unexamined Patent Publication No. 11-126947) will be described hereinafter.
FIGS. 8A and 8B show the most typical method for forming an ohmic electrode in an n-type electron channel. As shown in FIG. 8A, an n-type aluminum gallium nitride (AlxGa1-xN: 0≦x≦1) layer 32 serving as an active layer is formed on a sapphire substrate 31. A multilayer metal thin film 33 is formed on the AlxGa1-xN layer 32 by lift-off to include titanium (Ti) as the lowest layer and aluminum (Al), nickel (Ni), gold (Au), and the like thereon. Next, annealing is performed at a high temperature (500° C. through 900° C.). In this way, as shown in FIG. 8B, Ti of the multilayer metal thin film 33 is allowed to react with nitrogen (N) of the AlxGa1-xN layer 32 to form vacancies of nitrogen (N), thereby forming a region 34 with an increased metallicity in the vicinity of the top surface of the AlxGa1-xN layer 32. At the same time, Ga, Ti, TiN, Al, Ni, Au, and the like existing at the interface between the region 34 and the multilayer metal thin film 33 are allowed to react with one another to form a stable metal compound serving as an ohmic electrode.
With this method, the tunnel effect at the interface between the electron channel and an electrode metal is increased so that the contact resistance of the ohmic electrode can be reduced. For example, when source/drain electrodes of a typical heterojunction transistor obtained by stacking a 25-nm-thick Al0.25Ga0.75N layer and GaN are formed by this method, an ohmic electrode having a contact resistance of approximately 1×10−5 Ω·cm2 can be formed.
As the other method for forming an electrode in a Group III-V nitride semiconductor, a method using metal diboride is known which is intended to improve the lattice match between a Group III-V nitride semiconductor layer and an electrode (see, for example, Japanese Unexamined Patent Publication No. 2003-101038).
However, a contact resistance of the order of 10−6 through 10−7 Ω·cm2 is demanded for ohmic electrodes of practical high-frequency devices. The ohmic electrodes obtained by the known method for forming an ohmic electrode cannot satisfy this demand.
Since in the known example nitrogen vacancies are produced in a Group III-V nitride semiconductor layer to increase the tunnel effect at the interface between an electrode metal and a nitride semiconductor layer, this reduces the contact resistance of an ohmic electrode. In this relation, the lower limit of the contact resistance is defined by the height of a potential barrier located at the interface between a metal used for an ohmic contact and an electron-conducting channel, i.e., the barrier height.
The barrier height at the interface is determined by the work function of a metal used for an electrode. The work function represents the height from the Fermi level to the vacuum level. The work function of Ti usually used for electrodes in n−-type nitride semiconductors is approximately 5 eV. Accordingly, even when the tunnel effect is increased to reduce the contact resistance, it is extremely difficult to achieve a required contact resistance of the order of 10−6 through 10−7 Ω·cm2.
The present invention is made to solve the conventional problems, and its object is to realize a semiconductor device made of a Group III-V nitride semiconductor comprising a low-resistance ohmic electrode.